Soil Conservation Guidelines for Queensland
Chapter 7 Contour banks
Chapter 7
Contour banks
Key points
• Contour banks intercept runoff before it concentrates and starts to cause
erosion, and then safely channel it into stable grassed waterways, natural
depressions, or grassed areas adjacent to a paddock.
• When designing contour banks the key objective is to ensure that under the
design conditions the velocity of flow remains low enough to avoid erosion.
The velocity of flow depends on the gradient, length, spacing, and crosssection (or depth of flow) as well as the vegetation cover between, in, and on
the banks.
• The major considerations when designing contour banks are the land slope,
land use/cover, soil type and rainfall of the catchment. It is also important
to consider practical farm management requirements such as trafficability,
especially in choosing the spacing and alignment of the banks.
• Contour banks are usually designed to a standard that will safely carry runoff
resulting from a rainfall event with a 10 year average recurrence interval.
To ensure a safe operating margin, additional allowance is required for
freeboard and settlement following construction.
• Contour banks can be built in a range of different shapes (narrow-, broadbased, broad top- or bottom-side) depending on the slope, rainfall, soil
conditions, land use, and equipment available. Particular attention should
be given to the design and maintenance of the channel outlets as these are
weak points where erosion can often occur.
7–1
Contents
7.1Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
7.2 Contour bank types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
7.3
Design criteria. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
7.4 7.5 7.6
7.3.1
Design velocity. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
7.3.2 Gradients. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
7.3.3 Contour bank length. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
7.3.4 Bank spacing. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
7.3.5
Parallel layouts. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
7.3.6
Contour bank cross-sections. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
7.3.7 Freeboard and settlement. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
Design approach. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
7.4.1 Example. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
7.4.2
Contour bank design charts. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
Farming with contour banks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
7.5.1 Controlled traffic farming and ‘tramlining’. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
7.5.2 Contour banks and CTF layouts. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
7.5.3 CTF layouts on sloping land . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
7.5.4 Contour bank maintenance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
Further information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
Soil Conservation Guidelines for Queensland
Chapter 7 Contour banks
Glossary
average recurrence interval (ARI): the average period in
years between the occurrence of an event (usually a storm
or a flood) of specified magnitude and an event of equal
or greater magnitude.
contour bank: a constructed earth embankment,
incorporating a channel on the upslope side, typically
traversing a slope on or close to the contour to control
and/or prevent the erosion of that slope. Also referred to
as graded banks, terraces, or bunds.
controlled traffic farming (CTF): a cropping system in
which the crop zone and the machinery traffic lanes
are distinctly and permanently separated. In practice
it means that all implements have a particular span, or
multiple of it, and all wheel tracks are confined to specific
traffic lanes. Also referred to as tramlining.
flat ridge pattern: a variation in contour bank spacing that
arises when contour banks carry runoff across ridgelines
with low slopes or saddles.
freeboard: the vertical distance between the top water
level and the crest of a bank, dam, or similar structure.
Freeboard should include an allowance for settlement,
and is provided for in designing such structures to
prevent overtopping.
Manning’s roughness coefficient: see retardance.
Rational Method: a formula for estimating peak discharge
of runoff from a catchment above a specific point
calculated using the peak discharge, rainfall intensity for
the selected period, runoff coefficient, and catchment
area—see Chapter 4 of these guidelines.
retardance: a measure of resistance to flow in a channel;
the greater the resistance the higher the retardance. It
is calculated using the Manning’s formula and has the
symbol ‘n’. Retardance is influenced by the physical
roughness of the internal surface of the channel (e.g.
the vegetation that lines it), channel cross-section,
alignment, and obstructions.
stream power: the rate the energy of flowing water is
expended on the bed and banks of a channel.
Universal Soil Loss Equation (USLE): a mathematical
relationship developed in the USA to predict longterm average soil losses in runoff under specified
environmental and management systems.
ferrosols (krasnozems): soils with B2 horizons that are
high in free iron oxide, and which lack strong texture
contrast between A and B horizons.
7–3
7.1Introduction
Contour banks are earthen structures constructed across cultivated slopes at
intervals down the slope. In some countries and in some Australian states other
than Queensland, contour banks are referred to as ‘graded banks’, ‘terraces’,
or ‘bunds’. The function of contour banks is to intercept runoff and safely
channel it into stable grassed waterways, natural depressions, or grassed areas
adjacent to a paddock. They reduce slope length and intercept runoff before it
concentrates and starts to cause erosion. Contour banks also trap sediment from
overland flow, especially from rills and old gully lines. Any crop or stubble in a
contour bank channel acts as a filter as runoff moves slowly along the contour
bank channel.
These days, there is a tendency for people to downplay the importance of
contour banks as a soil control measure in cropping lands. Best management
practices for cropping land will often exclude contour banks, but will refer to
controlled traffic farming as being an alternative. In fact, the need for contour
banks, used in association with practices like zero tillage, is as great as ever. The
use of controlled traffic farming in association with contour banks is discussed in
section 7.5.
Contour bank layouts require careful planning to ensure that runoff is well
coordinated between properties within a catchment and across public utilities
such as roads and railway lines (see Figure 7.1). More information on how to
coordinate the planning of contour banks is provided in Chapter 2.
Figure 7.1: Plan of a contour bank and waterway layout
Contour banks are not built strictly ‘on the contour’. They need to have a low
gradient (usually between 0.1% and 0.4%) to allow water to flow but to minimise
the chance of channel flow reaching erosive velocities when the channel is
in a bare condition. In some intensive farming situations (e.g. horticulture or
sugarcane cultivation) where pondage must be avoided or where parallel layouts
are required, contour banks may be constructed to steeper gradients for limited
distances. If the channel is protected from erosion, for example, by maintaining
permanent cover, contour banks can be safely constructed at steeper gradients.
The spacing between contour banks depends mainly on the slope of the land but
is also influenced by soil type, cropping practices, and previous occurrence of
erosion at the site.
7–4
Soil Conservation Guidelines for Queensland
Chapter 7 Contour banks
Contour banks are usually designed in theory to a standard that will safely carry
water resulting from a runoff event with a 10 year average recurrence interval.
However, in reality, the ability of a contour bank to carry this design runoff is
largely dependent on the condition of the channel at the time the runoff event
occurs. A contour bank with a smooth, bare channel can carry around five times
more runoff than one where the channel supports a close growing crop or dense
stubble.
Cropping practices that maintain good surface cover of vegetation will greatly
reduce the incidence and amount of erosion between contour banks. This will
enhance the effectiveness of contour banks and greatly reduce their maintenance
costs.
Contour banks also play an important additional soil conservation role by acting
as sediment traps. Up to 80% of the soil moved from a contour bay may be
deposited in the contour bank channel (Freebairn and Wockner 1986). The rate of
deposition and filtration of nutrients and pesticides is greatest when the channel
contains a close growing crop or standing stubble.
In intensively cropped areas, such as where horticulture crops or sugar cane are
grown, contour banks are usually constructed parallel to each other. This is for
ease of inter-row cultivation, pesticide application, irrigation and harvesting
practices. However in broadacre cropping areas contour banks are usually
not laid out in parallel because of the irregular nature of the topography. The
introduction of controlled traffic farming over recent decades has also had
significant implications for contour bank systems. More information on contour
banks in horticulture is provided in Chapter 12.
Special techniques are required to construct contour banks in areas with shallow
dispersible subsoils (e.g. at depths of less than 30 cm). This is because when
such subsoils are exposed to rainfall and runoff in the channel, the contour bank
will be prone to failure by tunnel erosion.
7–5
7.2 Contour bank types
Contour banks can be constructed in four design styles:
Narrow-based contour banks (Figure 7.2) feature batters that are too steep
to cultivate. The batters of narrow-based contour banks are instead normally
planted to grass and hence require weed control, especially during the first two
years. The channels of narrow-based contour banks are usually treated as part
of the contour bay, which normally means that they are cultivated and cropped.
However, some farmers choose to leave the channels grassed. The channel and
batters of narrow-based contour banks may take up to 10% of the total area of a
paddock.
Figure 7.2: Narrow-based contour bank
original ground level
Narrow-based contour banks are commonly used on cultivated land that is
steep (i.e. 5–12% slopes) or only occasionally cultivated. They are not suited to
cracking clay soils because the deep cracks formed by these soils during the dry
season weaken the banks and may lead to subsequent failure. Narrow-based
contour banks are also prone to failure due to burrowing by animals where such
animals (e.g. rabbits) occur.
Broad-based contour banks (Figure 7.3) feature batters that can be easily worked
with tillage and planting machinery. This allows the whole of the paddock to
be cropped including the channel. Broad-based contour banks are generally
used on deep soils and on land with low slope (<5%). They can be safely crossed
by farming equipment under a controlled traffic system at a range of angles
depending on the slopes of their batters. Because the batters are cultivated, the
risk of failure of broad-based banks by cracking is reduced.
Figure 7.3: Broad-based contour bank
h
original ground level
Broad-based banks are more costly to build and maintain than narrow-based
banks, and become impractical to construct when slopes exceed about 5%.
Semi-broad-based banks may have a broad base either on the top side (Figure
7.4a) or the bottom side (Figure 7.4b). Broad-based top side banks are used on
steeper slopes and/or on cracking clay soils, where the up-slope batter of the
bank is broadened to suit the width of the most commonly used machinery.
Broad-based bottom side banks are commonly used on lower slopes, particularly
in irrigation areas—sugar cane—where the channel of the bank may be grassed
and used as an irrigator tow-path.
7–6
Soil Conservation Guidelines for Queensland
Chapter 7 Contour banks
Figure 7.4: Semi-broad-based contour bank a) top side, b) bottom side
h
h
(a)
(b)
original ground level
original ground level
7–7
7.3
Design criteria
Contour banks are normally not designed individually. Rather, general
specifications for contour banks are usually developed for a district and for
particular situations. These specifications are usually based on the following
parameters:
• gradient
• length
• spacing
• cross-section and depth of flow.
The capacity of contour banks generally declines over time. The height of
banks will reduce due to settlement soon after construction or progressive
wearing down due to tillage equipment, whilst the capacity of channels will also
progressively reduce due to sedimentation. Since contour bank maintenance
is normally infrequent (e.g. on a 5 to 10 year cycle) and irregular, it is best that
contour banks are constructed initially to exceed the specified standards by
a safe margin. That way, the banks will still function effectively, even as they
progressively deteriorate between maintenance events. Notwithstanding this,
it is also important to note that the dimensions of a newly constructed contour
bank are often governed by the construction technique rather than by the
prescribed specifications. For example, contour banks constructed with one
push of a large bulldozer (a commonly used technique) may greatly exceed the
standard specifications.
7.3.1 Design velocity
It is desirable that the flow velocity in a contour bank channel is low. This is to
avoid the chance of erosion and to ensure maximum deposition or trapping of
sediment. Low velocities also reduce design peak discharges in waterways by
lengthening the time of concentration. Where cultivated and cropped, the aim
of the contour bank design should be to keep velocities below 0.4 m/s for easily
eroded soils; and below 0.6 m/s for erosion resistant soils. Contour banks must
be designed with sufficient capacity to accommodate the design event at or
below this velocity.
The velocity of flow in a contour bank channel is very dependent on the condition
of the channel at the time that a runoff event occurs. If the channel is smooth
and unvegetated (i.e. Manning’s n of 0.03) the bank potential to discharge runoff
will be at a maximum. Under these conditions high velocities will occur if there
is a significant depth of flow such as in a major runoff event. However, if flow is
restricted by the presence of a cereal crop such as wheat or standing stubble
after harvest in the channel (i.e. where Manning’s n may be 0.15), the velocity is
not likely to exceed 0.2 m/s. Where controlled traffic farming systems are being
used, the crop rows may sometimes be at right angles to the direction of flow in
the channel. Under these circumstances, Manning’s n values could be expected
to be greater than 0.15. Further research is needed to determine what Manning’s
n values are likely to occur under these circumstances.
Because conditions that may occur in a contour bank channel are variable and
impossible to predict with absolute certainty, developing a design is complex
and requires acceptance of a level of risk. If a contour bank with a bare channel
is flowing to capacity, it is likely to be handling an event much greater than that
for which it was designed and erosive velocities will occur. This situation must be
deliberately risked, as the only alternative is to build a smaller bank or to reduce
the gradient. This would lead to regular failure if runoff events occur when the
channel is restricted by crop or stubble.
7–8
Soil Conservation Guidelines for Queensland
Chapter 7 Contour banks
If a design indicates that velocities in the channel will be too high, then the
following options should be considered:
• Use an alternative channel shape (e.g. a flat-bottomed trapezoidal shape will
convey flows more safely than a triangular cross-section).
• Keep the channel permanently grassed.
• Use a lower gradient.
The equation to calculate stream power (see Chapter 6) may be used to
determine the likelihood of erosion occurring in the channel. Table 7.1 provides
values of stream power for a typical broad-based contour bank with a trapezoidal
shape, a gradient of 0.2% and a Manning’s n of 0.03 (bare soil). For cracking clay
soils it is recommended that values of stream power be below 3 (W/m2) (Titmarsh
and Loch 1993). Table 7.1 indicates that this value will be exceeded for depths of
flow of 0.4 metres or greater for this design.
Table 7.1: Stream power values for a typical broad-based contour bank under bare soil conditions
Depth of flow (m)
Velocity (m/s)
0.2
0.4
0.3
0.5
0.4
0.6
0.5
0.7
0.6
0.79
0.7
0.87
Based on the following parameters:
Trapezoidal shape
Grandient of 0.2%
Manning’s n of 0.03
Discharge (m3/s)
0.5
1.1
1.8
2.9
4.2
5.8
Stream power (W/m2)
1.3
2.3
3.4
4.7
6.0
7.4
7.3.2 Gradients
The gradient of a contour bank design should be chosen to minimise the risk
of erosion in the channel when it is in a bare condition and also to ensure that
the channel has adequate capacity to carry the design runoff when flow in the
channel is restricted by crop or standing stubble. Such a compromise can be
difficult to achieve in practice because of the fivefold differences that can apply
in the values of Manning’s n (0.03 to 0.15) for these two situations.
Contour bank gradients that are too high result in:
• erosion in the contour bank channel
• excessive rates of runoff into waterways.
On the other hand, contour bank gradients that are too low lead to:
• poor drainage—an important issue, especially for many horticultural crops
• low points in the bank that will pond runoff until they are filled with sediment
• ‘leakage’ into groundwater systems in locations where this is an issue
• failure by ‘piping’ (linked to tunnel erosion) where there are dispersible
subsoils.
The impact of gradient on contour bank velocity and discharge is illustrated in
Figures 7.5a and 7.5b respectively.
7–9
Figure 7.5: Effect of gradient for two flow depths on a) contour bank velocity and b) contour bank discharge
contour bank velocity (metres / second)
1.2
(a)
Parameters:
Trapezoidal shape
1:10 inlet and 1:6 bank slope
4 m bed width
Mannings n of 0.03
1.0
0.8
0.6
0.4
Flow depth 0.5m
Flow depth 0.25 m
0.2
0.0
0.1
0.2
0.3
0.4
contour bank gradient (percentage)
Contour bank discharge (cubic metres / second)
4.5
4.0
(b)
Trapezoidal shape
1:10 inlet and 1:6 bank slope
Bed width 4 m
3.5
3.0
2.5
Flow depth 0.5 m
Flow depth 0.25 m
2.0
1.5
1.0
0.5
0.0
0.1
0.2
0.3
Contour bank gradient (percentage)
7–10
0.4
Soil Conservation Guidelines for Queensland
Chapter 7 Contour banks
The gradient recommended for a contour bank depends on the steepness of
the land and on the soil erodibility. This is in part because the capacity of a
contour bank of a given height depends on the land slope. The lower the land
slope the greater the storage capacity of the bank. Land slope also influences
contour bank length. Where the landscape is steep, the distance across the slope
between natural drainage lines is less. This means that the average distance that
contour banks are required to span between drainage lines on steep slopes is
likely to be much shorter than on low slopes.
Taking the above factors into account, there are good reasons for increasing
contour bank gradients as land slope increases. Steeper gradients on higher
slopes will compensate for the limited capacity of contour banks on such slopes.
However, shorter contour banks on steeper slopes means that they are required
to handle less runoff than longer banks, thus reducing the risk of erosion
occurring in the channel.
On horticultural properties, higher gradients can be used where the channel
is grassed, or where it is bare but not cultivated. If contour banks are used
for access and are not cultivated, the risk of erosion in the channel is greatly
reduced. In cane lands, gradients as high as 4% are used where green cane
trash blanketing is used on erosion-resistant soils [for example, ferrosols
(krasnozems)]. Contour bank channels in cane lands are only vulnerable to
erosion for a relatively short period, when a new crop is planted after the removal
of the ratoon crop (which occurs every 4 to 8 years). The use of minimum tillage
practices or a cover crop can reduce the risk of erosion during the fallow period.
Further information about managing contour bank gradients in cane lands is
included in section 7.3.5, Parallel layouts.
It is normal practice for a contour bank to be constructed to the same height
for its entire length. Since the amount of runoff to be carried increases with the
length of the contour bank, variable gradients can be used along a contour bank
channel, allowing for a greater discharge capacity. This will lengthen the time of
concentration and reduce the peak discharge in the waterway.
Where contour banks are on low land slopes with maximum gradients of less
than 0.2%, there is limited opportunity to use variable gradients. However, on
steeper land slopes where the maximum gradient is higher, variable gradients
can be used. As indicated in Table 7.2, this can mean that for a land slope of
3–5%, the gradient in the top third of the bank would be 0.2%, changing to
0.25% in the middle third and then to 0.3% in the lower third (outlet) section of
the bank.
Table 7.2: Gradients for contour banks with cultivated channels based on land slope
Land slope
1%
2%
3%–5%
5%–10%
Appropriate contour bank gradients (%)
for average conditions
Top section
Middle section
Outlet section
0.15
0.15
0.15
0.2
0.2
0.2
0.2
0.25
0.3
0.3
0.4
0.5
In intensive cropping areas, parallel contour bank systems are often
implemented (refer to section 7.3.5). This is for practicality in operating large
cropping machinery, and often in conjunction with incorporating an irrigation
layout. Some flexibility is required in setting contour bank gradients when
using a parallel contour bank system. Nevertheless, gradients should always be
managed to ensure that erosion in the channel is minimised.
7–11
Gradients can be modified over short distances to improve workability of the
layout. At the high end of a contour bank, it is quite acceptable to improve
workability by using a high or low gradient to ensure that the bank meets a fence
line at close to a right angle rather than an acute angle.
It is normal practice to ‘split’ contour banks on well-defined ridgelines so that
they direct runoff away from the ridge. This ensures that runoff remains in its
natural catchment and also provides an ideal position for a road or track to
cross over contour banks. The exact location of the ‘split’ should be prominently
marked during the surveying process so that the farmer is aware of its location
and the significance of this position. The splits on a ridge of a series of banks
down the slope should be aligned. This may require a readjustment of some
levels at the completion of the surveying task to obtain the best alignment.
If contour banks carry runoff across ridgelines that have low slopes or even a
saddle, considerable variation in the contour bank spacing (referred to as the
‘flat ridge pattern’) may result. This problem can be minimised by modifying
the gradient where the bank crosses the ridge. Some zero grade sections in
this situation would be acceptable as the low slope ensures maximum contour
bank capacity and the convex nature of the topography ensures that there is
less likelihood of concentrated flows discharging into this section of the contour
bank.
The land slope in an individual contour bay in many inland cropping areas may
vary from say 1% in the depression, to 0.5% on the ridge, to 2% in the area
between the ridge and the depression. (If there is a saddle, the ridge slope will
change from downhill to uphill). If this slope variation is not taken into account,
the surveyor may end up with highly embarrassing variations in the vertical
interval between contour banks. Consider the example below (see Figure 7.6
a–c).
Figure 7.6: Contour bank layout illustrating the flat ridge pattern: a) natural contour lines; b) banks flowing left to right;
c) banks flowing right to left
7–12
Soil Conservation Guidelines for Queensland
Chapter 7 Contour banks
Figure 7.6a shows the true contour lines on a ridge with slopes varying from
0.8% to 2.0%. Figure 7.6b shows what would happen if banks with a 0.2%
gradient were marked out in this paddock with the staffer moving in the direction
B to D. Suppose the contour bank vertical interval (VI) at AB was an acceptable
1.5 metres with a horizontal interval (HI) of 75 metres. By the time the staffer
reached point D, the VI at CD would be an unacceptable 0.96 metres (HI of only
48 metres).
The reason for this phenomenon is that the distance along the contour from B
to D in Figure 7.6b is 370 metres (giving a height change of 0.74 metres at 0.2%
gradient) while it is only 100 metres from A to C (giving a height change of 0.2
metres at 0.2% gradient).
Figure 7.6c shows what would happen if the staffer moved in the same direction
as in Figure 7.6b, but where the direction of flow was reversed. The VI would
change from 1.5 metres at AB to 2.53 metres at CD (HI at CD would be 127 metres).
The change in VI is even greater than it was in Figure 7.6b because the distance
from B to D is now 615 metres.
A similar situation (though not usually as severe) can occur in depressions.
What is the solution to this problem? The easiest solution, and maybe the
preferred option, is to split the banks on the ridge. This is a common and
sensible solution. However, splitting banks on every ridge may require more
waterways, and paddocks will be more difficult to work on the contour because
of the extra turning required.
Another solution is to reduce the gradient as the bank goes around the ridge. It
should be acceptable to reduce the gradient down to .05% because the low slope
on the ridge will mean that the contour bank will have a very high capacity. In
addition, as the slope on the ridge is always a convex one, there is not likely to
be any concentrated overland flow meeting the bank in this area. A gradient such
as .05% will cause much less variation in the vertical interval.
Where a contour bank crosses a ‘sharp’ depression, resulting in a sharp bend in
the bank, the gradient can be modified to smooth out the shape of the bank to
improve workability. However, this will create a low point in the contour bank that
will detain runoff until sufficient sedimentation occurs to remove the pond. If this
procedure is adopted, the contour bank must be constructed with extra capacity
where it crosses the drainage line. These points must also be checked after
construction is completed to ensure they have adequate capacity.
Consideration should be given to increasing gradients where contour banks
are to be built in land with prominent rilling and gullying. Alternatively, or in
addition, in such circumstances contour banks should be constructed with
additional height where they cross gully lines, bearing in mind that greater
settlement of the bank is likely to occur at these points. Additional height at
these points should reduce the need to increase the gradient. Ideally, gullies
will have been filled in during the construction process, although some form of a
depression is likely to remain. This depression will be subject to sedimentation
and will disappear over time. Levelling of the contour bay between contour banks
to remove old rill and gully lines is encouraged. If levelling is not carried out,
rills will continue to concentrate runoff from the adjacent area leading to silt
deposition in the contour bank channel.
Higher gradients can be considered for contour bays where zero tillage is
adopted or where contour bank channels are not cultivated. As previously
discussed the highest velocity likely to be achieved in a standard-size, broadbased contour bank with a wheat crop or standing wheat stubble is 0.2 m/s.
There is a risk however, that should the property change hands, a new owner
7–13
may adopt traditional cropping practices with lower levels of stubble and higher
velocities. Therefore, it is preferable to use gradients applicable to a farming
system that will have both bare and vegetated channels at different times.
From a hydraulic design aspect, level (0% gradient) contour banks, especially
on low slopes, could accommodate the runoff they receive, provided they were
built to an adequate specification. However, this is not recommended because
such banks are subject to pondage along the bank, inhibiting crop growth and
restricting tillage, planting and harvesting activities.
Gradients at contour bank outlets
Problems can occur at the point where contour banks discharge into waterways.
In this situation, the gradient of the surface of the water in a channel is an
important consideration as well as the gradient in the bed of the contour bank
channel (Stephens 1987). Two different situations may occur: (1) where a bank
discharges with a completely free outlet, and (2) where the outlet is obstructed
in some way.
Where a bank discharges with a completely free outlet
Examples of where a bank discharges with a completely free outlet include where
it discharges into:
• a wide deep hollow
• an adjacent grass paddock
• a subsurface waterway
• an eroding waterway.
In the above cases, the gradient of the water surface would be greater than that
of the channel, and the velocity of the flow would increase as it discharges. This
can cause erosion in bank outlets. In these situations there is no requirement for
any extra gradient at the bank outlet.
Where contour banks are discharging into a grassed area, it is advisable to
construct a spreader channel (Figure 7.7) at the outlet to ensure that discharge
occurs over a wide section of the bank. Spreader channels are level channels
created by pushing soil uphill rather than downhill as with conventional contour
banks. They are used to reduce the concentration of water discharging at the end
of a diversion or contour bank into an area of pasture or a watercourse.
Figure 7.7: Plan view of a spreader channel at the outlet of a contour bank
A spreader channel would normally require the last 20–50 metres of a contour
bank to be level. This section would have an excavated channel from which soil
has been pushed uphill. A hedge incorporating a species such as Monto vetiver
grass along the spreading area will help ensure that runoff exits the sill over the
entire length of the spreading area.
Where there is an overfall at the potential outlet of a contour bank, some
adjustment to contour bank spacing may be required to find a more stable outlet
7–14
Soil Conservation Guidelines for Queensland
Chapter 7 Contour banks
for a contour bank. Normal gradients or even a level section should be used
where there is an overfall. Such overfalls should be stabilised at the outlet by
means of a structure such as a rock chute. Where the contour bank outlet is
unstable, the last section of channel should be permanently grassed.
Where the outlet is obstructed in some way
Examples of where the discharge is obstructed in some way include:
• a bank outlet that is too narrow or choked with grass or stubble
• where the bank discharges into a waterway that is flowing at a similar height
to the water in the contour bank.
In the above cases the gradient of the water surface in the channel of the contour
bank will be less than that of the channel bed and the velocity will decrease.
This can cause the bank to overflow near the outlet. The contour bank gradient
will generally need to be increased in these situations to provide for both the
estimated depth of excavation to construct the contour bank as well as the
design depth of flow above ground level in the waterway.
In situations where the ground slope is low it may not be possible to sufficiently
increase fall at the bank outlet. In such situations bank height should be
increased for at least the last 200 metres of the contour bank. As an additional
measure to reduce risk of overflow, the contour bank may be constructed to
discharge into a secondary waterway running adjacent to the main waterway for
about half a contour bay interval.
7.3.3 Contour bank length
Farmers generally prefer contour banks to be as long as possible and waterways
to be as few as possible to minimise loss of cropped area and to reduce
interference with normal practices using large cropping machinery. However this
can come at a cost, as bank length increases so does the risk of failure.
Contour bank length is related to land slope. On steeper landscapes, natural
drainage lines are closer together, meaning that the distances to be spanned
by contour banks are shorter. Whilst, on lower slopes the capacity of a bank of
a given height will be greater than on a steeper slope (see Section 7.3.6). This
means that longer banks and lower gradients can be used on low slopes. The
longest bank lengths are implemented on low sloping extensive cropping areas
of the Western Downs and the Central Highlands. Contour banks may however be
shortened in more intensive cropping systems such as with the growing of sugar
cane or other horticultural crops for ease of harvesting.
The amount of runoff discharged from a contour bay will be proportional to the
area of the bay. Figure 7.8 predicts peak discharges for various contour bank
lengths and different levels of cover on a 2% slope with a 90 metre contour bank
spacing, at Pittsworth on the Darling Downs, using the Empirical version of the
Rational Method—see Chapter 4. Significantly higher discharge is predicted
under the low cover system due to the higher velocity (and hence shorter time
of concentration) and the higher C value. A contour bank with bare soil in the
channel will be able to accommodate considerably more runoff than a bank with
a channel that is carrying a crop or standing stubble.
Table 7.3 provides a guide to selecting maximum bank lengths based on land
slope. This table assumes contour bank capacities normally maintained by
farmers on such slopes and the minimum contour bank spacing normally
recommended on such slopes. It also assumes that the runoff is travelling in one
direction in the contour bank channel.
7–15
Figure 7.8: Peak discharge estimates based on contour bank length at Pittsworth
Table 7.3 : Recommended maximum bank lengths for various land slopes
Land slope %
Recommended maximum bank length (m)
1
2500
1.5
2000
2
1750
3
1500
4
1000
5
750
6
600
7
450
8
400
9
350
10
300
Based on the following parameters:
Single spaced contour banks
Use of cropping systems that provide high levels of cover
High standard of contour bank maintenance
An alternative approach to designing of contour bank length is to use the KINCON
model (Connolly et al. 1991).
7–16
Soil Conservation Guidelines for Queensland
Chapter 7 Contour banks
7.3.4 Bank spacing
Farmers prefer wide spacing between contour banks as it ensures less
interference with the operation of farm machinery and reduces the cost per
hectare of construction. However there are a number of factors that limit the
spacing between contour banks:
• Wider spacing increases erosion.
• Wider spacing allows overland flows to concentrate, forming gullies between
the banks and building up deltas in the channel of the contour bank below.
• There are practical limitations on bank size and on a bank’s ability to handle
runoff.
Various guides have been developed to determine contour bank spacings.
These are based on land slope, soil erodibility, land use, and rainfall erosivity.
These factors are not all equally significant. The Universal Soil Loss Equation
(see Chapter 1) shows for instance that steepness of slope has a much more
significant impact on erosion than does the length of the slope.
There are no strict rules that determine the ‘correct’ spacing for a particular
situation. The concept of ‘single’ and ‘double’ spacing has been used to vary
contour bank spacing depending on the average conditions expected in a
paddock. Experience in Queensland has shown that the spacing listed in Table
7.4 is suitable for most cropping situations.
Table 7.4: Recommended contour bank spacing
Average land
Single spacing
Double spacing
slope (%)
Vertical interval (VI)(m) Horizontal interval (HI)(m) Vertical interval (VI)(m) Horizontal interval (HI)(m)
1
0.9
90
1.8
180
2
1.2
60
2.4
120
3
1.4
45
2.8
90
4
1.6
40
3.2
80
5
1.8
36
3.6
72
6
1.9
32
3.8
64
7
2.1
30
4.2
60
8
2.4
30
4.8
60
9
2.7
30
5.4
60
10
3.0
30
6.0
60
Single spacing should be used where:
• bare fallow cropping systems are likely to be used
• a paddock is already seriously eroding
• soils are highly erodible
• contour bank length is close to the recommended maximum length
• contour banks are likely to be maintained only to a minimum standard
• parallel contour banks with higher than normal gradients are planned.
Double spacing may be used where:
• cropping systems that ensure high stubble levels during the fallow phase are
used
• minimal erosion has occurred and soils are stable
• contour banks are likely to be built and maintained to a high standard.
7–17
Intermediate spacing (between single and double) is used in some districts. An
argument against this practice is that if the spacing were subsequently halved it
would result in spacing that is unacceptably close for most farmers. Experience
has shown that the normal single or double spacing is acceptable provided the
conditions listed above are met.
Other factors may determine the spacing required for a particular situation. For
example, parallel contour banks in irrigated cane are traditionally spaced 40
metres or 80 metres apart to match the spray width of water winches used for
irrigation.
Where topography is irregular, the distance between banks will also vary with
changes in the slope of the land. For this reason it is preferable to measure
bank spacing using the vertical interval rather than the horizontal interval. To
determine the vertical interval in areas of variable topography a compromise is
required. The recommended approach is to use the average vertical interval for
the contour bay.
7.3.5 Parallel layouts
Parallel layouts are required for any situation where inter-row farming operations
are practised or where crops are irrigated. Parallel layouts have been used
traditionally in intensive cropping areas such as for sugar cane or horticulture.
Implementing parallel layouts requires additional detailed topographic
information. Parallel layouts are most readily implemented where the topography
is regular (i.e. slope varies minimally within each of the proposed contour bays).
In intensive cropping areas, contour banks are deliberately shortened allowing
for greater opportunities to alter gradients to ensure that the contour bank
system is parallel.
The implementation of a parallel layout usually relies on using as many natural
depressions as possible. This will result in many short contour banks with
consequent negative impact on machinery manoeuvrability by requiring the
operator to turn around at each waterway. One option to overcome this problem
is to use subsurface waterways, (see Chapter 9). Subsurface waterways assist
trafficability by allowing the tractor operator to lift an implement and travel
across the waterway.
Using single spaced contour banks in parallel layouts will reduce the amount
of runoff that the channels need to accommodate. This will also provide more
options for varying gradients to implement the parallel system. The spacing
should be modified to match the implement widths or the irrigation system in
use on the farm.
Where higher than normal gradients are required, the use of a parabolic, or a
flat-bottomed, contour bank channel rather than a triangular one should be
considered. It may also be necessary that the channel be grassed. Designs
should be tested thoroughly before completion to be sure that the expected
velocities are not likely to cause erosion when the channels are in a bare
condition.
Table 7.5 (Scarborough et al. 1992) provides examples of gradients
recommended for use in parallel layouts in the Coastal Burnett region. This
table applied to situations where contour bank channels are cultivated but could
be used as a general guide for the whole of Queensland. If green cane trash
blanketing is used and measures are taken to provide erosion protection after
the removal of the ratoon crop (every 4 to 8 years), then higher gradients than
shown in Table 7.5 could be used.
7–18
Soil Conservation Guidelines for Queensland
Chapter 7 Contour banks
Table 7.5: Recommended gradients for contour banks in parallel layouts where channels are cultivated (from
Scarborough et al. 1992)
Soil erodibility
Low
Medium
High
Average grade (%)
1.5
1.0
0.5
Max. grade for 50 m (%)
3.0
2.0
1.0
A steep gradient of say 3–4% may be acceptable over a short distance, for
example 50 metres, at the high end of a contour bank channel. This is because
minimal flow is being carried in this section.
It is often a great advantage if contour banks are not only parallel to each other
but also straight. This is especially the case in vineyards or with trellised tree
crops. It is difficult and expensive to build trellises on curved alignments.
Straight parallel banks are also preferred in canelands, as water winches used to
irrigate sugar cane generally require straight rows to operate effectively.
Some sections of reverse grade may be unavoidable in a parallel contour bank
system. Such sections will however cause ponding. The extent to which ponding
may be a problem depends on the cropping system used and the soil types
present. It may be possible to avoid having a reverse grade by constructing an
additional cut in the elevated section of the channel. Alternatively, a low section
leading to a reverse gradient can be corrected by constructing that section of the
bank from the lower side. This will result in the channel at that point being higher
than adjacent sections.
Parallel layouts have seldom been implemented on broadacre farming
systems. This is because the rolling landscapes of such areas normally feature
considerable slope variation. The lowest slopes are usually found on ridge lines
while the maximum slopes occur between the ridge line and the drainage line.
Contour banks used in broadacre cropping are generally long with low gradients.
There is limited opportunity to increase the gradient unless the contour bank
channel is to be permanently grassed.
Controlled traffic farming (CTF) systems that have been widely adopted in recent
decades require land to be cultivated in parallel blocks. In broadacre systems,
this has generally been achieved by cultivating whole paddocks, usually in a
single direction, passing up and over contour banks (see Section 7.5). In the
South Burnett region, some farmers have implemented parallel farming with
non-parallel, broad-based contour banks by selecting a key bank and working
parallel to it. Contour banks above and below the key bank are then crossed at
a slight angle. This systems results in furrows that are close to the contour but
which drain either into a waterway or a contour bank.
7.3.6 Contour bank cross-sections
Contour banks can take a range of shapes as discussed at the beginning of this
chapter. While contour banks are commonly constructed with a trapezoidal
shape, the cross-section usually reverts to a triangular shape after a few years of
tillage operations.
The cross-sectional area of a contour bank is most influenced by the bank height
and the land slope. Figure 7.9 illustrates the effect of bank height on the crosssectional area of flow. The data is based on a triangular-shaped, broad-based
contour bank on a 2% land slope and with a bank batter of 1:6 (V:H). It assumes
that the excavated upslope batter conforms to normal land slope.
7–19
Figure 7.9: Effect of bank height on the cross-sectional area of a contour bank
Figure 7.10 illustrates the effect that land slope has on contour bank capacity.
As for Figure 7.9, the data is based on a triangular-shaped, broad-based bank
with a flow depth of 0.5 metres and a bank batter of 1:6. It also assumes that the
excavated batter conforms to normal land slope. On a land slope of 1% where a
contour bank vertical interval of 0.9 metres would be used, half the contour bay
would be under water if there was a flow depth of 0.45 metres. This illustrates
the enormous amount of storage that contour banks can have on very low slopes.
Figure 7.10: Effect of land slope on contour bank cross-sectional area
Where land slopes are low, the excavated batter will often conform to the
normal land slope after a few years of tillage operations. If the bank has been
constructed with a bulldozer using a long length of travel in pushing up the bank,
then the excavated bank batter will almost conform with normal land slope after
construction is complete.
7–20
Soil Conservation Guidelines for Queensland
Chapter 7 Contour banks
Figure 7.11 also illustrates how land slope impacts on the cross- sectional area of
contour banks. Five percent (5%) land slope is the normally accepted limit for the
construction of broad based contour banks with 1:4 (V:H) batters.
Figure 7.11: Broad-based contour banks with 1:4 batters on land slopes 1%, 2%, and 5%
(a) 1% slope
1% slope
1:4
1:4
0.63 m
5m
5m
1:33
5m
45 m
(b) 2% slope
2% slope
1:4
1:4
5m
0.63 m
5m
1:17
5m
15 m
(c) 5% slope
pe
5% slo
1:4
1:4
5m
0.63 m
5m
1:7
5m
To provide protection against erosion of contour bank channels on steeper
slopes, it is best to aim for a flat-bottomed channel (trapezoidal or parabolic).
On steeper slopes there will be a distinct change in slope where the excavated
batter meets the normal land slope. This point is referred to as the ‘nick
point’ (Figure 7.12). It can contribute to rill erosion as overland flows meet the
increased slope as they flow into the channel.
Figure 7.12: Contour bank cross-section illustrating nick point
nick point
The limitations (and requirements) of machinery must be taken into
consideration when determining contour bank cross-sections. The length
and grade of the batters of contour banks should be constructed to suit the
equipment used to operate on them (especially planting machinery).
For cultivated banks, batters flatter than 1:4 (V:H) are recommended. Section
7.5 provides information on contour bank shapes suitable for traversing by
machinery. If a trapezoidal channel is constructed then the base must also
conform to machinery requirements. Tow paths for travelling irrigators located
in the bank channel require a trapezoidal shape with at least a 2.0 metre bottom
width to help tracking of the irrigator.
7–21
7.3.7 Freeboard and settlement
Following construction, contour bank capacities need to be checked to ensure
they are adequate for the conditions. Special attention should be given to
checking points where contour banks cross old gully lines. The bank should
be built higher at these locations to ensure that it has adequate capacity to
accommodate the design flow as it crosses the old gully line.
7–22
Soil Conservation Guidelines for Queensland
Chapter 7 Contour banks
7.4 Design approach
Equation 7.1
R0.66 S0.5
Q
=V=
n
A
Where
Q = the discharge or hydraulic
capacity of the channel (m3/s)
A = cross sectional area (m2)
V = average velocity (m/s)
R = hydraulic radius (m)
S = channel slope (m/m)
n = Manning’s coefficient of
roughness.
As stated at the beginning of this chapter, contour banks are normally
constructed according to general specifications that are then applied to
particular circumstances across a whole district. Such specifications have been
developed after numerous field observations over many years. In some instances
this information can be found in the relevant land management field manual.
When it is necessary to develop an individual design for a contour bank
or to prepare or modify district specifications, the following approach is
recommended.
In Chapter 6 the concept of combining equations 6.1 (Q=AV) and 6.2 (the
Manning formula) was discussed—see Equation 7.1.
Because the surface conditions of a channel in a contour bank may vary from a
bare condition (Manning’s n of 0.03) to a crop or standing stubble (Manning’s
n of 0.15 in the case of a wheat crop or standing wheat stubble), it is necessary
to consider both conditions in the design. This requires estimating two design
discharges. The design peak discharge for a contour bank varies considerably
for a high- and low-cover farming system (see Figure 7.8). The amount of cover
varies between and across all paddocks and through time. A low-cover paddock,
as would result from a fallow management farming system, will have a high
cover level when it is growing a crop. A paddock where a high level of stubble
management is used may have low cover during a period of drought when no
crop has been planted or the crop has failed.
Since flow in contour banks is most restricted under crop or standing stubble it
is best to design initially for this condition and then check to see what happens
to the design discharge when the channel is bare. A limitation of this method
of design is that it does not take the temporary storage capacity of the contour
bank channel into account. The method therefore provides an overestimation
of the actual capacity required. In addition, contour banks can act as temporary
storage structures (Galletly 1980). Further research is required to develop a
design method that incorporates storage capacity.
Factors in Equation 7.1 for which values are known can be directly substituted as:
• discharge, Q
• gradient, S
• roughness coefficient, n.
Since the design is initially for a high level of channel roughness, it can be
assumed that the flow will be well below erosive velocities and therefore the
value of V can be omitted from the equation. Solving the equation then resolves
to finding a depth of flow in the contour bank channel that will give a hydraulic
radius, R, and cross-sectional area, A, that will accommodate the required value
of Q for a given gradient and value of Manning’s n. This can be solved using an
iterative process.
The best way to undertake this iterative process is to prepare a spreadsheet
based on the required cross sectional shape incorporating trial depths of flow
and a high and low value of Manning’s n (Table 7.6). It can be seen that erodible
velocities (>0.5 m/s) will occur once the depth of flow in a bare channel exceeds
0.25 m depth of flow. However when the channel is protected by standing
stubble, a flow depth of 0.7 m will only be flowing at 0.17m/s.
7–23
Table 7.6: Discharges and velocities for a range of flow depths for a trapezoidal-shaped contour bank with a gradient of
0.2%
Manning’s n = 0.15
Manning’s n = 0.03
e.g. standing wheat stubble
e.g. bare cultivated channel
Velocity (m/s) Discharge (m3/s) Velocity (m/s) Discharge (m3/s)
0.10
0.48
0.09
0.06
0.03
0.29
0.14
0.15
0.78
0.12
0.07
0.06
0.37
0.29
0.20
1.12
0.15
0.09
0.10
0.44
0.49
0.25
1.50
0.19
0.10
0.15
0.49
0.74
0.30
1.92
0.22
0.11
0.21
0.54
1.04
0.35
2.38
0.25
0.12
0.28
0.59
1.41
0.40
2.88
0.28
0.13
0.37
0.64
1.83
0.45
3.42
0.30
0.14
0.46
0.68
2.32
0.50
4.00
0.33
0.14
0.58
0.72
2.88
0.55
4.62
0.36
0.15
0.70
0.76
3.50
0.60
5.28
0.39
0.16
0.84
0.80
4.20
0.65
5.98
0.41
0.17
0.99
0.83
4.97
0.70
6.72
0.44
0.17
1.16
0.87
5.82
Bold colour indicates erosive velocities for a bare soil (>0.5 m/s)
Parameters:
Trapezoidal cross-section with inlet slope of 1:10, bank batter of 1:6 and bed width of 4 metres
Contour bank gradient of 0.2%
Depth (m)
Cross-sectional
area (m2)
Hydraulic
radius (m)
7.4.1 Example
To determine the constructed height for a contour bank to accommodate
discharges of 0.4 m3/sec when a contour bay has a mature wheat crop (n = 0.15)
and a discharge of 0.9 m3/sec when the contour bay is under bare fallow (n =
0.03). The contour bank is to have a trapezoidal cross-section with inlet batters
of 1:10, bank batter of 1:6, bed width of 4 metres and a gradient of 0.2%. Assume
that the bank will be built by a bulldozer and that it will settle by 50% after
construction.
Solution:
Step 1. Use a spreadsheet to prepare a table similar to Table 7.6 showing
velocities and discharges for the two values of n for a range of trial depths and
for the assumed gradient, i.e. 0.2%.
Step 2. From Table 7.6 when n = 0.15 a flow depth of 0.4 m will have a discharge
of 0.37 m3/sec with a velocity of 0.13 m/sec.
Step 3. From Table 7.6 when n = .03 a flow depth of 0.3 m will have a discharge
of 1.04 m3/sec with a velocity of 0.54 m/sec. (Note that there is a risk of an
erodible velocity for the bare soil condition).
Step 4. Table 7.6 shows that a depth of flow of 0.4 m would be sufficient to
accommodate the required flow. (Note that if an alternative design was
required an additional spreadsheet could be prepared based on a different
gradient.)
Step 5. An allowance of 0.15 m for freeboard would give a recommended settled
bank height of 0.55 m which can accommodate the mature crop condition.
Step 6. An additional 50% should be added to allow for settlement giving a
constructed height of 1.1 m (Table 6.3, in Chapter 6).
Note: should the bank carry a design depth of flow of 0.4 m in a bare fallow
condition, Table 7.6 shows that it would be carrying a discharge of 1.83 m3/sec at
a velocity of 0.64 m/sec. Such a velocity is likely to be erosive but such an event
7–24
Soil Conservation Guidelines for Queensland
Chapter 7 Contour banks
would be rare as it is double the design discharge for bare fallow. Since bare
fallow farming systems contribute to high rates of soil erosion in a contour bay,
it is most desirable that a high-cover farming system is adopted rather than one
that has bare fallows.
7.4.2 Contour bank design charts
Design charts can be prepared to show how contour banks with a specified
cross-section perform under a range of values of Manning’s n, gradient and flow
depth. Figure 7.13 is an example of a contour bank design chart for a broad-based
contour bank with a bottom width of 4 metres and batters of 1:6 and 1:10 (V:H).
The three graphs illustrate the dramatic effect that surface roughness in the
channel has on both velocity and discharge.
Figure 7.13: Contour bank design chart for a trapezoidal shape and a range of values for Manning’s n, channel gradient
and flow depth
1.2
1
1
1.1
10
6
1.0
CONTOUR BANK CROSS SECTION
0.9
0.7
0.7
d=
d=
0.6
m
0.4
d=
s=
%
0.4
0.5
0.5
.2%
0.
1m
.1%
0.2
0.1
0.2
0.3
0.4
0.6
0.8
1.0
2.0
1.5
3
2.5
3.0
5.0
4.0
6.0 7.0
d
0.2
8.0
=
2m
0.
0.7
d=
m
m
1%
s = 0.
m
d
.1
=0
0.1
0.06
0.1
0.2
0.3
0.4
0.6
0.8
1.0
1.5
2.0
2.5
3.0
4.0
5.0
0.2
d
0.1
=
3m
0.
= s = 0.4%
d
%
s = 0.3
2m
0.
d
=
4m
0.
d
=
d
=
=
6m
0.
d
5m
0.
0.
7m
3
0.3
s = 0.2%
s = 0.1%
1m
d = 0.
0.0
0.5
=
d
d
=
s=0
d
0.
3m
0.3
0.3
=
0
s=
d=
m
s=
0.2
%
0.2
d=
s=
%
0.4
%
0.3
0.
4m
0.4
0.6
s=
0.4
d=
d=
0.3
.3
=0
m
m
%
s
d=
0.5
m
0.6
0.7m
0.8
0.6m
0.8
0.06
0.1
0.3
0.2
0.4
0.6
0.8
1.0
1.5
2.0
CONTOUR BANK
DESIGN CHARTS
3
26/7/99
7–25
Figure 7.14 shows a graph for the same cross-section as Figure 7.13 but for a
constant gradient of 0.2% and three values of Manning’s n.
Figure 7.14: Contour bank design chart for a trapezoidal shape and a range of values for Manning’s n and flow depth
1
1
10
6
CONTOUR BANK CROSS SECTION
1.2
1.1
1.1
1.0
1.0
d=0.7m
1.2
d=0.6
m
0.9
d=0.4
m
0.7
0.6
d=0
0.6
.2m
.05
n=0
0.5
d=0
0.5
0.9
0.8
.3m
0.7
03
0.
d=0.5
m
0.8
n=
0.4
0.4
0.3
0.3
0.2
0.2
n=0.15
0.1
0.1
0.0
0.06
s = 0.002m/m = 0.2%
Mannings n
Bare cultivated channel n = 0.03
Channel with sparce grass cover n = 0.05
Channel with standing wheat stubble n = 0.15
0.1
0.2
0.3
0.4
0.6
0.8
3
1.0
1.5
2.0
2.5
3.0
4.0
5.0
6.0 7.0
0.0
8.0
CONTOUR BANK
DESIGN CHARTS
BANK GRADIENT = 0.2%
6/1/99
7–26
Soil Conservation Guidelines for Queensland
Chapter 7 Contour banks
7.5 Farming with contour banks
Most contour bank systems in Queensland were constructed many years ago.
They were designed to be cropped by working the bays along the contour parallel
to the channels. This was comparatively simple with the smaller machinery and
more flexible farming systems in common use 30 years or so ago. However, it has
become increasingly difficult as agriculture has scaled up, become much more
mechanised, and adopted new systems such as zero till and controlled traffic
farming. These recent changes mean that in some instances we need to rethink
how a paddock containing contour banks should be worked.
7.5.1 Controlled traffic farming and ‘tramlining’
Controlled traffic farming (CTF) began in Queensland in the early 1990s. CTF is a
system where all field traffic including the harvester is restricted to permanent
wheel tracks referred to as traffic lanes. Global positioning systems (GPS) are
often used to steer the machinery and keep it on the precise track. With ‘true
CTF’ all machinery is modified to have the same wheel base so that the wheels
of each piece of equipment travel over the exact same ground. ‘Tramlining’
is similar to CTF in that all pieces of equipment travel in the same lanes (i.e.
parallel to a fence line) but with tramlining the wheel base of the various pieces
of equipment varies so that their wheels do not necessarily travel over the exact
same ground.
When a paddock is trafficked in the traditional manner by a variety of tractors,
harvesters, implements and trucks with different wheel spacings, a considerable
amount of soil compaction occurs in all areas of the paddock. With CTF (and to a
lesser extent tramlining) this compaction is restricted to just the limited parallel
wheel tracks. CTF has mostly been applied to broadacre cropping areas but is
also being adopted in sugar cane and horticulture.
When combined with zero tillage, CTF can result in the following advantages:
• reduced overall compaction (especially when it is required that paddocks are
trafficked when in a moist condition, as for example, when harvesting)
• more porous soils in the cropping bays, allowing movement of water, air, plant
roots, and soil organisms and producing healthier plants ensuring higher
yields
• more efficient farming operations, as minimal overlap and longer runs result
in up to 25% reduction of fuel, seed, fertiliser, and chemical usage
• energy savings because of minimal overlap, tyres moving on ‘permanent’
compacted wheel tracks and tines working in uncompacted soil. This results
in reduced greenhouse gas emissions.
7.5.2 Contour banks and CTF layouts
Contour banks help to control erosion on sloping land by intercepting runoff
before it concentrates and then channelling it to a safe disposal area such as a
grassed waterway. The role of contour banks becomes particularly important in
seasons when there is minimal ground cover.
Most contour banks are not parallel. This leads to inefficiencies and compaction
when working along the contour between contour banks. It is now a common
practice for farmers operating on slopes of less than 4% to work their paddocks
perpendicular to, and crossing over, the contour banks, and often parallel to a
fence line or block boundary—a practice referred to as ‘tramlining’.
7–27
It should be noted that in most instances this practice has been adopted for
convenience and is not necessarily part of a fully implemented CTF system.
When working a paddock parallel to a fence line as shown in Figure 7.15, the
direction of wheel tracks and planting rows may vary considerably in relation
to the slope. In some parts of the paddock, tracks will be virtually up and down
slope whilst in other parts they will be generally across the slope. This means
that any runoff flowing along and down the rows and tracks may concentrate
and cause erosion in various parts of the paddock. If contour banks are spaced
widely apart this problem will be accentuated. The risk of erosion will be
particularly high where drainage lines have been eliminated by contour banks
(‘contoured out drainage lines’). Figure 7.15 contains an example of such a
drainage line.
Figure 7.15: Tramlines parallel to a fence line
7.5.3 CTF layouts on sloping land
When implementing CTF, the aim is to keep runoff in the same wheel track or
crop row as it flows downhill. If this can be achieved it will avoid runoff being
concentrated and causing erosion. With traditional contour cultivation, runoff
is often discharged into contour banks by rills. The resultant silt ‘slug’ can lead
to waterlogging in the contour bank channel and bank failure may occur where
banks are below specifications. With CTF, wheel tracks and crop rows will
discharge their runoff evenly along the entire length of contour banks.
To avoid the situations represented in Figure 7.15, a CTF layout needs to be
designed with the following principles in mind:
• Wheel tracks and crop rows should intersect contour banks at as near as
possible to right angles (90°). Lower angles of intersection (down to as low as
45°) may be acceptable provided there is no significant rilling in the paddock.
• A herringbone pattern (see Figure 7.16) should be used on water-spreading
areas such as ridges (divergent drainage).
• Additional waterways or drains may be required where runoff concentrates
(convergent drainage), as shown in Figure 7.16.
Figure 7.16 shows an example of a CTF layout on a relatively flat (2%) slope.
Contour banks that directed all runoff to the waterway on the edge of the
paddock had been constructed prior to adoption of CTF. For the CTF layout, two
additional waterways have been added to accommodate convergent drainage.
7–28
Soil Conservation Guidelines for Queensland
Chapter 7 Contour banks
Figure 7.16: A CTF layout with two herringbone patterns
Original grassed waterway
Additional grassed waterway
Contour bank
Ridge line
Wheel track direction
500 metres
The layout has two herringbone patterns and the layouts pivot on headlands on
ridge lines and on either side of waterways. Access tracks could be incorporated
along the ridgeline headlands, below the contour, or beside the watercourse, as
shown in Figure 7.17.
Runoff from wheel tracks and furrows must be able to flow into either a contour
bank or a waterway. In some cases it may be necessary to construct spur banks
between contour banks as shown in Figure 7.17. These banks direct runoff into
waterways through gaps in the waterway bank. Runoff directed to the waterway
would cause erosion if it was not allowed to enter the waterway via the spur
bank.
Figure 7.17: Spur banks allowing runoff from wheel tracks and furrows to enter the waterway
7–29
Most CTF layouts in grain-growing areas have been applied to land with slopes
that are 3% or less. On steeper slopes, it is progressively more difficult to
construct and maintain the broad batters on contour banks that enable tractors,
implements, and harvesters to traverse them. Figure 7.18 shows how direction of
working can be varied between contour bays depending on the land slope. In this
example, bays 1 and 2 with a 2% slope are worked up and down hill. Bays 3 and
4 with a 3% slope are worked parallel to the contour banks.
Figure 7.18: Parallel strips of cultivation between non-parallel contour banks on 3% slopes
As machinery traversing contour banks can reduce their capacity, it is important
to monitor them regularly and to carry out maintenance when required.
7.5.4 Contour bank maintenance
Poorly maintained contour banks are a liability and are likely to cause rather than
prevent erosion. When a bank breaks, the outflow may cause severe erosion in
the contour bay below and contribute to the failure of subsequent contour banks.
Erosion occurs spasmodically. It is not possible to predict when a ‘rogue’ storm
is likely to occur, so contour banks must always be maintained to the correct
capacity. A property manager should not get complacent about contour bank
maintenance during a period of dry years when there is little runoff. In any 10year period a few of the biggest storms—even in one year—may cause 80 to 90%
of the total soil loss for that 10-year period.
Sediment deposited in contour banks may contribute to ponding at various
points along a contour bank channel. Such ponding, as well as increasing the risk
of the bank overtopping, can be costly to farmers as it inhibits planting, weed
control and harvesting operations.
Contour bank capacity
Recommended contour bank capacity depends on many factors. The amount of
runoff a contour bank has to carry depends on the length of the contour bank and
the bank spacing (see sections 7.3.3 and 7.3.4).
Land slope has a big influence on bank capacity. A bank on low sloping land will
be able to store much more runoff than a bank of the same height on steeper
land. Contour bank capacity will gradually reduce over time. Although they may
appear to be suitable, contour banks could be dangerously lacking in capacity if
they have not been maintained for several years.
7–30
Soil Conservation Guidelines for Queensland
Chapter 7 Contour banks
Table 7.7 shows the effect of depth on the capacity of a typical broad-based
contour bank channel with a small amount of stubble in the channel. As depth of
flow increases, there is a marked increase in the velocity of flow in the channel
and the contour bank is able to carry much higher amounts of runoff.
Table 7.7: Effect of depth of flow on the capacity of a contour bank
Depth of flow in the
contour bank channel
(m)
Conditions in the contour bank channel
Crop or standing stubble
0.3
0.2
1.0
0.4
0.4
1.8
Bare fallow
Discharge (m3/sec)
0.5
0.6
2.9
0.6
0.8
4.2
These results are based on a contour bank cross section and a contour bank
gradient of 0.2%:
Factors that reduce bank capacity
Bank capacity may be reduced in the following ways:
• Sheet and rill erosion in the interbank area deposits sediment in the channel.
Soil loss experiments on the Darling Downs have shown that paddocks with
inadequate levels of surface cover may average soil losses of around 50
tonnes of soil per year. However around 80% of this soil may be deposited in
the contour bank channel.
• Soil tends to move downhill during cultivating and ploughing.
• Tillage operations move soil downhill and flatten bank crests.
• Vehicle and animal tracks reduce bank height.
How to check capacity
Contour bank capacity can be checked by using a length of string and a line level
as shown in Figure 7.19. Alternatively, a builder’s level can be used on a straight
piece of timber.
Figure 7.19: Checking contour bank capacity with a line level
X
Line level
Ruler
String line
Y
h
Cross sectional area, A(m2), is calculated by multiplying one half of length XY
(metres) by bank height (h) (metres)—Equation 7.2.
Under opportunity cropping there may be minimal chances to carry out
maintenance works, so the opportunity should be taken when it arises.
The capacity of contour banks can also be captured by traversing the bank using
an RTK (real-time kinematic) GPS receiver on a tractor or vehicle. The logging
interval needs to be set very short (e.g. 1 point/sec), and driving done slowly up
and over the banks.
7–31
Equation 7.2
A (m2) = ½ X Y x h
For example:
h = 60 cm (0.6 m)
XY = 14 m
A = 4.2 m2
The resultant data gives a very good profile of the bank, from the comfort of the
vehicle. With a small amount of processing, the wetted perimeter and capacity
can then be calculated.
The first point for banks to fail will be any low spots in the contour bank
(comparable to a small dam). The weakest link in a contour bank is often a point
where the bank has been constructed across an old rill or gully line. Higher rates
of settlement can occur at such points and the effective height of the bank may
be much less than the average bank height. Such low points can be observed
visually by looking upwards at the bank from the middle of the contour bay
below.
While maintenance is being carried out, landholders should check for low
sections in contour or diversion banks. Low spots are easy to identify when
looking uphill so that banks can be seen silhouetted over the top of the ones
below, when low places are immediately evident. These should be filled and
made higher to allow for settling.
Figure 7.20: Checking for low sections by looking uphill
Special attention should be paid to bank outlets as it is these sections that carry
the most water. Bank outlets are especially susceptible to failure if vehicles or
animals cross the bank at this point.
Contour banks should also be checked for any piping or tunnelling that may
be associated with cracks during a dry season, or animal burrows, and repairs
undertaken as soon as practical.
Reducing maintenance costs
The following measures have been found to reduce maintenance costs:
• Maintain high levels of surface cover (at least 30%).
• Level out rills and gully lines in the contour bay.
• If crossing over contour banks in a controlled traffic system, use implements
with adequate flexibility and ground clearance.
Maintaining banks
Sediment deposited in contour bank channels following high-intensity storms
should be removed as soon as practical. Scrapers are ideal for collecting
sediment from the channel and using it to fill the rilled areas above the channel.
For regular maintenance work, bank heights may be maintained by moving the
sediment out of the channel and onto the contour bank. This maintenance is
commonly carried out with a grader or dozer. A grader will work more efficiently
if the soil has been previously ripped or cultivated; it is also useful for cleaning
bank channels. A dozer blade or scraper is useful in repairing broken banks,
which should be repaired as soon as possible to prevent further damage.
7–32
Soil Conservation Guidelines for Queensland
Chapter 7 Contour banks
While many contour banks are constructed with a semi-trapezoidal shape, they
tend to become triangular in cross-section after a few years of cultivation, as
the upslope batter tends to be indistinguishable from the natural slope. The
triangular cross-section appears to be the easiest shape for farmers to maintain
and is most compatible with tillage and planting implements.
Bank capacity and maintenance in relation to CTF
Because of the need to traverse contour banks with tractors and harvesters with
controlled traffic farming, there is a tendency for farmers to have their contour
banks low as they believe it will be more practicable. However, the late 1990s
and early 2000s have generally been periods of below average runoff with
limited runoff events, and many farmers have become complacent about contour
bank maintenance. Many farmers who believe that their contour banks are
suitable for traversing actually have banks with inadequate capacity.
Tillage equipment traversing contour banks will reduce bank capacity by
‘dragging down’ the crest of the contour bank. Additionally, wheel tracks in wet
weather can be up to 15 cm deep. This effectively reduces contour bank height,
resulting in lowering the capacity of a contour bank. Table 7.8 indicates that for a
triangular-shaped contour bank on a 2% slope, the reduction in effective height
from 60 cm to 45 cm would reduce cross-sectional capacity by 44%, that is, from
10.1 m2 to 5.7 m2.
Table 7.8: Effect of contour bank height on cross-sectional area
Contour bank height (m)
Channel cross-sectional area (m2)
0.4
4.5
0.45
5.7
0.5
7.0
0.55
8.5
0.6
10.1
Parameters:
Land slope 2%
Bank batter 1:6
Upper bank batter conforms to the 2% land slope
Wheel tracks and press wheels on planters can also lead to the development of
cracks as soils dry out. Such cracks can lead to bank failure if they fail to seal up
before a runoff event occurs.
The risk of failure of contour banks under a controlled traffic layout will be
greater on steeper slopes where a contour bank of a given height will have much
less capacity than it will on a low slope. If banks are very low, it may be beneficial
to hire a contractor to bring them up to specifications, but usually, a few extra
runs with a disc plough along the bank will do the job.
Emergency maintenance
Any breaks that occur should be mended as soon as possible, even if it
means running over an area of crop. Equally important are deposits of silt that
sometimes occur when rows break over in cultivated crops. If not removed, these
will partially block channels or outlets and could cause failure of the whole
system. A dozer, bucket loader or grader blade is the best type of equipment to
use for this job.
7–33
7.6
Further information
References
Connolly, RD, Silburn, DM, and Barton, N (1991) Design
of long contour banks in south-west Queensland.
Information series No: QI91014, Department of Primary
Industries, Queensland.
Freebairn, DM and Wockner, GH (1986) A study of soil
erosion on vertisols of the eastern Darling Downs,
Queensland, II , The effect of Soil, Rainfall, and Flow
Conditions on Suspended Losses. Australian Journal of
Soil Research 24, 135–158.
Galletly, JC (1980) Design and efficiency of contour banks.
Agricultural Engineering Conference, Geelong, 1980.
Scarborough, R, Stone, B, and Glanville, T (1992)
Specifications for erosion control measures. In: Land
Management Manual, Coastal Burnett Districts,
Queensland Department of Primary Industries, Brisbane.
Stephens, RM (1987) Water flow at the outlets of contour
banks, Division of Land Utilisation, Department of Primary
Industries, Brisbane.
Titmarsh, G and Loch, R (1993) Towards more efficient
soil conservation layouts, report to the Cotton Research
and Development Corporation, Department of Primary
Industries, Queensland.
Other information
For further information on contour banks and managing
erosion on cropping lands consult the Queensland
Government website <qld.gov.au/environment/land/soil/
erosion/management/> and the following fact sheets:
• Erosion control in cropping land
• Runoff control measures for erosion control in cropping
land
• Controlled traffic farming—Soil conservation
considerations for extensive cropping
• Maintaining contour banks
• Contour bank specifications.
7–34